Residual coding for transform skip mode and block differential pulse-code modulation

ABSTRACT

A method of video decoding performed in a video decoder is provided. A bit stream including bins of syntax elements is received. The syntax elements correspond to coefficients of a region of a transform skipped block in a coded picture. The syntax elements include a first flag indicating whether an absolute coefficient level of one of the coefficients is greater than a first threshold value, and a second flag indicating a parity of the absolute coefficient level. The second flag is decoded in a pass. The pass satisfies at least one of: (1) no other syntax elements is decoded in the pass; (2) a third flag indicating whether the absolute coefficient level is greater than a second threshold value is decoded in the pass; and (3) a fourth flag indicating sign information of the coefficient level of the one of the coefficients is decoded in the pass.

INCORPORATION BY REFERENCE

This present application claims the benefit of priority to U.S.Provisional Application No. 62/809,677, “Improved Residual Coding forTransform Skip Mode and Block Differential Pulse-Code Modulation” filedon Feb. 24, 2019, which is hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer thebits that are required at a given quantization step size to representthe block after entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is only using reference data from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). A predictor block can be formed using neighboring sample valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or mayitself be predicted.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar motion vector derived from MVs of aneighboring area. That results in the MV found for a given area to besimilar or the same as the MV predicted from the surrounding MVs, andthat in turn can be represented, after entropy coding, in a smallernumber of bits than what would be used if coding the MV directly. Insome cases, MV prediction can be an example of lossless compression of asignal (namely: the MVs) derived from the original signal (namely: thesample stream). In other cases, MV prediction itself can be lossy, forexample because of rounding errors when calculating a predictor fromseveral surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding,” December 2016).

Referring to FIG. 1, a current block (101) comprises samples that havebeen found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (102 through 106, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for videocoding at a decoder. In some examples, a bit stream including bins ofsyntax elements is received. The syntax elements correspond tocoefficients of a region of a transform skipped block in a codedpicture. The syntax elements include a first flag indicating whether anabsolute coefficient level of one of the coefficients is greater than afirst threshold value, and a second flag indicating a parity of theabsolute coefficient level. The second flag is decoded in a pass. Thepass satisfies at least one of: (1) no other syntax elements is decodedin the pass; (2) a third flag indicating whether the absolutecoefficient level is greater than a second threshold value is decoded inthe pass; and (3) a fourth flag indicating sign information of thecoefficient level of the one of the coefficients is decoded in the pass.The second threshold value being greater than the first threshold value.

In an embodiment, the first threshold value is 1 and the secondthreshold value is 3.

In an embodiment, a current block corresponding to the transform skippedblock is coded with a block differential pulse-code modulation mode.

In an embodiment, the second flag is decoded in the pass withoutdecoding other syntax elements after decoding the first flag and a fifthflag indicating whether the absolute coefficient level is greater than 3in a previous pass.

In an embodiment, the second flag is decoded in the pass withoutdecoding other syntax elements after decoding the third flag in aprevious pass.

In an embodiment, the second flag is decoded in the same pass with thethird flag.

In an embodiment, the second flag is decoded in the same pass with thefourth flag.

In some example, a bit stream including bins of syntax elements isreceived. The syntax elements correspond to coefficients of a region ofa transform skipped block in a coded picture. The syntax elementsinclude a first flag indicating whether an absolute coefficient level ofone of the coefficients is greater than a first threshold value, and asecond flag indicating whether the absolute coefficient level of the oneof the coefficients is greater than a second threshold value. The secondthreshold value is greater than the first threshold value. A number ofthe bins of the second flag that are context coded is determined basedon coded information of the coefficients. Context modeling is performedto determine a context model for each of the number of the bins of thesecond flag. The number of the bins of the second flag is decoded basedon the determined context models.

In an embodiment, the first threshold value is 1 and the secondthreshold value is 3.

In an embodiment, the coded information of the coefficients includes anumber of the first flag in a current coefficient group (CG) includingthe coefficients, a number the first flag in a previous CG, whether acurrent block corresponding to the transform skipped block is intracoded or inter coded, a size of the transform skipped block, a width ofthe transformed skipped block, a height of the transformed skippedblock, or an aspect ratio of the transformed skipped block.

In an embodiment, when the current block is intra coded, the number ofthe bins of the second flag is more than the number of the bins of thesecond flag when the current block is inter coded.

In some examples, a bit stream including bins of syntax elements isreceived. The syntax elements correspond to coefficients included in acoefficient group (CG) of a transform skipped block in a coded picture.A maximum number of context coded bins of the CG or a maximum averagenumber of context coded bins of the CG is determined based on codedinformation of the coefficients. Context modeling is performed todetermine a context model for each of a number of the bins of syntaxelements in the CG. The number of the bins of syntax elements in the CGbeing context coded does not exceed the maximum number of context codedbins of the CG or the maximum average number of context coded bins ofthe CG. The number of the bins of syntax elements is decoded based onthe determined context models.

In an embodiment, the coded information of the coefficients includes anumber of the first flag in the CG including the coefficients, a numberthe first flag in a previous CG, whether a current block correspondingto the transform skipped block is intra coded or inter coded, a size ofthe transform skipped block, a width of the transformed skipped block, aheight of the transformed skipped block, or an aspect ratio of thetransformed skipped block.

Aspects of the disclosure also provide non-transitory computer-readablestorage mediums storing instructions which when executed by a computercause the computer to perform any of the above methods.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of a current block and itssurrounding spatial merge candidates in one example.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 6 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 7 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 8A shows examples of block differential pulse-code modulation(BDPCM) coded blocks in accordance with an embodiment.

FIG. 8B shows examples of BDPCM coded blocks in accordance with anembodiment.

FIG. 9A shows an exemplary context-based adaptive binary arithmeticcoding (CABAC) based entropy encoder in accordance with an embodiment.

FIG. 9B shows an exemplary CABAC based entropy decoder in accordancewith an embodiment.

FIG. 10 shows an example of a sub-block scan order in accordance with anembodiment.

FIG. 11 shows an example of a sub-block scanning process from whichdifferent types of syntax elements of transform coefficients aregenerated in accordance with an embodiment.

FIG. 12 shows an example of a local template used for context selectionfor current coefficients in accordance with an embodiment.

FIG. 13A shows an example of the context used for coding the signinformation of a current coefficient in accordance with an embodiment.

FIG. 13B shows an example of the context used for coding the signinformation of a current coefficient in accordance with an embodiment.

FIG. 14 shows a flow chart outlining a coefficient decoding process inaccordance with an embodiment.

FIG. 15 shows a flow chart outlining a coefficient decoding process inaccordance with an embodiment.

FIG. 16 shows a flow chart outlining a coefficient decoding process inaccordance with an embodiment.

FIG. 17 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

I. Video Coding Encoder and Decoder

FIG. 2 illustrates a simplified block diagram of a communication system(200) according to an embodiment of the present disclosure. Thecommunication system (200) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (250). Forexample, the communication system (200) includes a first pair ofterminal devices (210) and (220) interconnected via the network (250).In the FIG. 2 example, the first pair of terminal devices (210) and(220) performs unidirectional transmission of data. For example, theterminal device (210) may code video data (e.g., a stream of videopictures that are captured by the terminal device (210)) fortransmission to the other terminal device (220) via the network (250).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (220) may receive the codedvideo data from the network (250), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (200) includes a secondpair of terminal devices (230) and (240) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (230) and (240)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (230) and (240) via the network (250). Eachterminal device of the terminal devices (230) and (240) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (230) and (240), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 2 example, the terminal devices (210), (220), (230) and(240) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (250) represents any number ofnetworks that convey coded video data among the terminal devices (210),(220), (230) and (240), including for example wireline (wired) and/orwireless communication networks. The communication network (250) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(250) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 3 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (313) that caninclude a video source (301), for example a digital camera, creating forexample a stream of video pictures (302) that are uncompressed. In anexample, the stream of video pictures (302) includes samples that aretaken by the digital camera. The stream of video pictures (302),depicted as a bold line to emphasize a high data volume when compared toencoded video data (304) (or coded video bitstreams), can be processedby an electronic device (320) that includes a video encoder (303)coupled to the video source (301). The video encoder (303) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (304) (or encoded video bitstream (304)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (302), can be stored on a streamingserver (305) for future use. One or more streaming client subsystems,such as client subsystems (306) and (308) in FIG. 3 can access thestreaming server (305) to retrieve copies (307) and (309) of the encodedvideo data (304). A client subsystem (306) can include a video decoder(310), for example, in an electronic device (330). The video decoder(310) decodes the incoming copy (307) of the encoded video data andcreates an outgoing stream of video pictures (311) that can be renderedon a display (312) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (304),(307), and (309) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (320) and (330) can includeother components (not shown). For example, the electronic device (320)can include a video decoder (not shown) and the electronic device (330)can include a video encoder (not shown) as well.

FIG. 4 shows a block diagram of a video decoder (410) according to anembodiment of the present disclosure. The video decoder (410) can beincluded in an electronic device (430). The electronic device (430) caninclude a receiver (431) (e.g., receiving circuitry). The video decoder(410) can be used in the place of the video decoder (310) in the FIG. 3example.

The receiver (431) may receive one or more coded video sequences to bedecoded by the video decoder (410); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (401), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (431) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (431) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (415) may be coupled inbetween the receiver (431) and an entropy decoder/parser (420) (“parser(420)” henceforth). In certain applications, the buffer memory (415) ispart of the video decoder (410). In others, it can be outside of thevideo decoder (410) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (410), forexample to combat network jitter, and in addition another buffer memory(415) inside the video decoder (410), for example to handle playouttiming. When the receiver (431) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (415) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (415) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (410).

The video decoder (410) may include the parser (420) to reconstructsymbols (421) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (410),and potentially information to control a rendering device such as arender device (412) (e.g., a display screen) that is not an integralpart of the electronic device (430) but can be coupled to the electronicdevice (430), as was shown in FIG. 4. The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (420) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (420) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (420) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (420) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (415), so as tocreate symbols (421).

Reconstruction of the symbols (421) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (420). The flow of such subgroup control information between theparser (420) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (410)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (451). Thescaler/inverse transform unit (451) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (421) from the parser (420). The scaler/inversetransform unit (451) can output blocks comprising sample values, whichcan be input into aggregator (455).

In some cases, the output samples of the scaler/inverse transform (451)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (452). In some cases, the intra pictureprediction unit (452) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (458). The currentpicture buffer (458) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(455), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (452) has generated to the outputsample information as provided by the scaler/inverse transform unit(451).

In other cases, the output samples of the scaler/inverse transform unit(451) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (453) canaccess reference picture memory (457) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (421) pertaining to the block, these samples can beadded by the aggregator (455) to the output of the scaler/inversetransform unit (451) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (457) from where themotion compensation prediction unit (453) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (453) in the form of symbols (421) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (457) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (455) can be subject to variousloop filtering techniques in the loop filter unit (456). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (456) as symbols (421) from the parser (420), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (456) can be a sample stream that canbe output to the render device (412) as well as stored in the referencepicture memory (457) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (420)), the current picture buffer (458) can becomea part of the reference picture memory (457), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (410) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (431) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (410) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 5 shows a block diagram of a video encoder (503) according to anembodiment of the present disclosure. The video encoder (503) isincluded in an electronic device (520). The electronic device (520)includes a transmitter (540) (e.g., transmitting circuitry). The videoencoder (503) can be used in the place of the video encoder (303) in theFIG. 3 example.

The video encoder (503) may receive video samples from a video source(501) (that is not part of the electronic device (520) in the FIG. 5example) that may capture video image(s) to be coded by the videoencoder (503). In another example, the video source (501) is a part ofthe electronic device (520).

The video source (501) may provide the source video sequence to be codedby the video encoder (503) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (501) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (501) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (503) may code andcompress the pictures of the source video sequence into a coded videosequence (543) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (550). In some embodiments, the controller(550) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (550) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (550) can be configured to have other suitablefunctions that pertain to the video encoder (503) optimized for acertain system design.

In some embodiments, the video encoder (503) is configured to operate ina coding loop.

As an oversimplified description, in an example, the coding loop caninclude a source coder (530) (e.g., responsible for creating symbols,such as a symbol stream, based on an input picture to be coded, and areference picture(s)), and a (local) decoder (533) embedded in the videoencoder (503). The decoder (533) reconstructs the symbols to create thesample data in a similar manner as a (remote) decoder also would create(as any compression between symbols and coded video bitstream islossless in the video compression technologies considered in thedisclosed subject matter). The reconstructed sample stream (sample data)is input to the reference picture memory (534). As the decoding of asymbol stream leads to bit-exact results independent of decoder location(local or remote), the content in the reference picture memory (534) isalso bit exact between the local encoder and remote encoder. In otherwords, the prediction part of an encoder “sees” as reference picturesamples exactly the same sample values as a decoder would “see” whenusing prediction during decoding. This fundamental principle ofreference picture synchronicity (and resulting drift, if synchronicitycannot be maintained, for example because of channel errors) is used insome related arts as well.

The operation of the “local” decoder (533) can be the same as of a“remote” decoder, such as the video decoder (410), which has alreadybeen described in detail above in conjunction with FIG. 4. Brieflyreferring also to FIG. 4, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (545) and the parser (420) can be lossless, the entropy decodingparts of the video decoder (410), including the buffer memory (415), andparser (420) may not be fully implemented in the local decoder (533).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (530) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (532) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (533) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (530). Operations of the coding engine (532) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 5), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (533) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (534). In this manner, the video encoder(503) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (535) may perform prediction searches for the codingengine (532). That is, for a new picture to be coded, the predictor(535) may search the reference picture memory (534) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(535) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (535), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (534).

The controller (550) may manage coding operations of the source coder(530), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (545). The entropy coder (545)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (540) may buffer the coded video sequence(s) as createdby the entropy coder (545) to prepare for transmission via acommunication channel (560), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(540) may merge coded video data from the video coder (503) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (550) may manage operation of the video encoder (503).During coding, the controller (550) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (503) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (503) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (540) may transmit additional datawith the encoded video. The source coder (530) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 6 shows a diagram of a video encoder (603) according to anotherembodiment of the disclosure. The video encoder (603) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (603) is used in theplace of the video encoder (303) in the FIG. 3 example.

In an HEVC example, the video encoder (603) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (603) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (603) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(603) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (603) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 6 example, the video encoder (603) includes the interencoder (630), an intra encoder (622), a residue calculator (623), aswitch (626), a residue encoder (624), a general controller (621), andan entropy encoder (625) coupled together as shown in FIG. 6.

The inter encoder (630) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (622) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (622) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (621) is configured to determine general controldata and control other components of the video encoder (603) based onthe general control data. In an example, the general controller (621)determines the mode of the block, and provides a control signal to theswitch (626) based on the mode. For example, when the mode is the intramode, the general controller (621) controls the switch (626) to selectthe intra mode result for use by the residue calculator (623), andcontrols the entropy encoder (625) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(621) controls the switch (626) to select the inter prediction resultfor use by the residue calculator (623), and controls the entropyencoder (625) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (623) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (622) or the inter encoder (630). Theresidue encoder (624) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (624) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (603) also includes a residuedecoder (628). The residue decoder (628) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (622) and theinter encoder (630). For example, the inter encoder (630) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (622) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (625) is configured to format the bitstream toinclude the encoded block. The entropy encoder (625) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (625) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 7 shows a diagram of a video decoder (710) according to anotherembodiment of the disclosure. The video decoder (710) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (710) is used in the place of the videodecoder (310) in the FIG. 3 example.

In the FIG. 7 example, the video decoder (710) includes an entropydecoder (771), an inter decoder (780), a residue decoder (773), areconstruction module (774), and an intra decoder (772) coupled togetheras shown in FIG. 7.

The entropy decoder (771) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (772) or the inter decoder (780), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (780); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (772). The residual information can be subject to inversequantization and is provided to the residue decoder (773).

The inter decoder (780) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (772) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (773) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (773) mayalso require certain control information to include the QuantizerParameter (QP), and that information may be provided by the entropydecoder (771) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (774) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (773) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (303), (503), and (603), and thevideo decoders (310), (410), and (710) can be implemented using anysuitable technique. In an embodiment, the video encoders (303), (503),and (603), and the video decoders (310), (410), and (710) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (303), (503), and (503), and the videodecoders (310), (410), and (710) can be implemented using one or moreprocessors that execute software instructions.

II. Transform Skip Mode

In an embodiment of the present disclosure, a transform skip (TS) modecan be applied for coding both intra and inter prediction residuals. Fora luma or chroma coding block with less than or equal to 16 samples, aflag may be signaled to indicate whether the TS mode is applied forcurrent block.

When the TS mode is applied, the prediction process is the same as theprediction process in the regular transform mode. In some examples,intra or inter prediction may be applied. For transform skipping TUs, ascaling process may be used so that transform skipping coefficients canhave similar magnitudes as other transform coefficients. In anembodiment, a scaling-down process may be performed, and the scalingfactor may be the same as the scaling associated with other transforms(versus standard floating point transform with norm 1) of the same size.Moreover, when the TS mode is applied, de-quantization and scaling arethe same as the de-quantization and scaling in the regular transformmode. Deblocking, sample adaptive offset (SAO), and adaptive loopfiltering (ALF) also are the same when the TS mode is applied, but aflag may be signaled to indicate whether transform is bypassed. Further,a flag may be signaled in the SPS to indicate whether the TS mode isenabled or not.

In an example, the related spec text of the TS mode in a VVC draft isdescribed in Table 1 below:

TABLE 1 Residual coding syntax Descriptor residual_coding( x0, y0,log2TbWidth, log2TbHeight, cIdx ) {  if( transform_skip_enabled_flag &&( cIdx ! = 0 | | cu_mts_flag[ x0 ][ y0 ] = = 0 )  && ( log2TbWidth <= 2) && ( log2TbHeight <= 2 ) )   transform_skip_flag[ x0 ][ y0 ][ cIdx ]ae(v)  last_sig_coeff_x_prefix ae(v)  last_sig_coeff_y_prefix ae(v)  if(last_sig_coeff_x_prefix > 3 )   last_sig_coeff_x_suffix ae(v)  if(last_sig_coeff_y_prefix > 3 )   last_sig_coeff_y_suffix ae(v) log2SbSize = ( Min( log2TbWidth, log2TbHeight ) < 2 ? 1 : 2 ) numSbCoeff = 1 << ( log2SbSize << 1 )  lastScanPos = numSbCoeff  ......

In Table 1, transform_skip_flag[x0][y0][cIdx] specifies whether atransform is applied to the associated transform block or not. Whentransform_skip_flag[x0][y0][cIdx] is equal to 1, it specifies that notransform is applied to the current transform block. Whentransform_skip_flag[x0][y0][cIdx] is equal to 0, it specifies thatwhether transform is applied to the current transform block depends onother syntax elements. When transform_skip_flag[x0][y0][cIdx] is notpresent, it is inferred to be equal to 0.

The array indices x0, y0 specify the location (x0, y0) of a top-leftluma sample of a transform block relative to a top-left luma sample of apicture. The array index cIdx specifies an indicator for the colorcomponent. When the array index cIdx is equal to 0, the color componentis luma. When the array index cIdx is equal to 1, the color component isCb. When the array index cIdx is equal to 2, the color component is Cr.

Next, an example of the scaling and the transformation process isdescribed below.

Inputs to the process are:

-   -   (a) a luma location (xTbY, yTbY) specifying a top-left sample of        a current luma transform block relative to a top left luma        sample of a current picture,    -   (b) a variable cIdx specifying the color component of the        current block,    -   (c) a variable nTbW specifying the transform block width, and    -   (d) a variable nTbH specifying the transform block height.

The output of this process may be the (nTbW)×(nTbH) array of residualsamples resSamples[x][y] with x=0 . . . nTbW−1, y=0 . . . nTbH−1.

In the scaling process, a variable bitDepth is a bit depth of thecurrent color component, a variable bdShift is a scaling shift factor,and a variable tsShift is a transform skip shift. The variablesbitDepth, bdShift, and tsShift may be derived as follows:

bitDepth=(cIdx==0)?BitDepth_(Y):BitDepth_(C)   (Eq. 1)

bdShift=Max(22−bitDepth, 0)   (Eq. 2)

tsShift=5+((Log2(nTbW)+Log2(nTbH))/2)   (Eq. 3)

The scaling process for transform coefficients may be invoked with thetransform block location (xTbY, yTbY), the transform width nTbW and thetransform height nTbH, the color component variable cIdx, and the bitdepth of the current color component bitDepth as the inputs, and an(nTbW)×nTbH) array of scaled transform coefficients d as the output.

The (nTbW)×nTbH) array of residual samples r is the quantizedcoefficients and may be derived as follows:

If transform_skip_flag[xTbY ][yTbY][cIdx] is equal to 1, the residualsample array values r[x ][y] with with x=0 . . . nTbW−1, y=0 . . .nTbH−1 may be derived as follows:

r[x][y]=d[x][y]<<tsShift   (Eq. 4)

When transform_skip_flag[xTbY][yTbY][cIdx] is equal to 0, thetransformation process for scaled transform coefficients may be invokedwith the transform block location (xTbY, yTbY), the transform width nTbWand the transform height nTbH, the color component variable cIdx and the(nTbW)×(nTbH) array of scaled transform coefficients d as the inputs,and an (nTbW)×(nTbH) array of residual samples r as the output.

The residual samples resSamples[x][y] with x=0 . . . nTbW−1, y=0 . . .nTbH−1 can be derived as follows:

resSamples[x][y]=(r[x][y]+(1<<(bdShift−1)))>>bdShift   (Eq. 5)

III. Block Differential Pulse-code Modulation Mode

Block differential pulse-code modulation (BDPCM) is an intra-coding toolthat uses a differential pulse-code modulation (DPCM) approach at theblock level. A bdpcm_flag may be transmitted at the CU level wheneverthe CU is a luma intra coded CU having each dimension smaller or equalto 32. This flag indicates whether regular intra coding or DPCM is usedand encoded using a single Context-based adaptive binary arithmeticcoding (CABAC) context.

BDPCM may use the Median Edge Detector of LOCO-I (e.g., used inJPEG-LS). Specifically, for a current pixel X having pixel A as a leftneighbor, pixel B as a top neighbor, and C as a top-left neighbor, theprediction of the current pixel X P(X) is determined by the followingformula:

$\begin{matrix}\begin{matrix}{{P(X)} = {\min \left( {A,B} \right)}} & {{{{if}\mspace{14mu} C} \geq {\max \left( {A,B} \right)}}\mspace{14mu}} \\{{\max \left( {A,B} \right)}} & {{{{if}\mspace{14mu} C} \leq {\min \left( {A,B} \right)}}\mspace{14mu}} \\{{A + B - C}} & {{otherwise}}\end{matrix} & \left( {{Eq}.\mspace{14mu} 6} \right)\end{matrix}$

The pixel predictor uses unfiltered reference pixels when predictingfrom the top row and left column of the CU. The predictor then usesreconstructed pixels for the rest of the CU. Pixels are processed in araster-scan order inside the CU. The prediction error may be quantizedin the spatial domain, after rescaling, in a way identical to thetransform skip quantizer. Each pixel can be reconstructed by adding thedequantized prediction error to the prediction. Thus, the reconstructedpixels can be used to predict the next pixels in the raster-scan order.Amplitude and signs of the quantized prediction error are encodedseparately. A cbf_bdpcm_flag is coded. If cbf_bdpcm_flag is equal to 0,all amplitudes of the block are to be decoded as zero. If cbf_bdpcm_flagis equal to 1, all amplitudes of the block are encoded individually inraster-scan order. In order to keep complexity low, the amplitude can belimited to at most 31 (inclusive). The amplitude may be encoded usingunary binarization, with three contexts for the first bin, then onecontext for each additional bin until the 12th bin, and one context forall remaining bins. A sign may be encoded in bypass model for each zeroresidue.

In order to maintain the coherence of the regular intra mode prediction,the first mode in the most probable mode (MPM) list of the intra modeprediction is associated with a BDPCM CU (without being transmitted) andis available for generating the MPM for subsequent blocks.

The deblocking filter may be deactivated on a border/boundary betweentwo BDPCM coded blocks because neither of the BDPCM coded blocksperforms a transform, which is usually responsible for blockingartifacts. Further, BDPCM may not use any other step except the onesdescribed here. In particular, BDPCM may not perform any transform inresidual coding as described above.

Several tests regarding BDPCM have been conducted to investigate thethroughput improvements of BDPCM and the interaction with other ScreenContent Coding (SCC) tools.

FIG. 8A shows examples of BDPCM coded blocks according to an embodiment.The examples shown in FIG. 8A are related to Test 1. As shown in FIG.8A, in order to increase throughput, smaller blocks (e.g., having sizesof 4×4, 4×8, and 8×4) may be divided into two independently decodableareas, using a diagonal which effectively divides the block into twohalves (e.g., stair-case shaped partition).

In an embodiment, pixels from one area of a first half may not beallowed to use pixels from another area of a second half to compute theprediction. If pixels from one area need to use pixels from another areato compute the prediction, reference pixels are used instead. Forexample, the pixels from the other area may be replaced by the closestreference pixels. For example, a left neighbor may be replaced with aleft reference pixel from the same row, a top neighbor may be replacedwith a left reference pixel from the same column, and a top-leftneighbor may be replaced with the closest reference pixel. Thus, the twoareas can be processed in parallel.

FIG. 8A also provides an exemplary throughput of each block having adifferent size. For example, for a 4×4 block with two independentlydecodable areas, the throughput may be 4 pixels per cycle. For a 4×8 or8×4 block with two independently decodable areas, the throughput may be5.33 pixels per cycle. For an 8×8 block without independently decodableareas, the throughput may be 4.26 pixels per cycle. For an 8×8 blockwithout independently decodable areas, the throughput may be 4.26 pixelsper cycle. For a 16×16 block without independently decodable areas, thethroughput may be 8.25 pixels per cycle.

FIG. 8B shows examples of BDPCM coded blocks according to an embodiment.The examples shown in FIG. 8B are related to Test 2. In FIG. 8B, theblock may be divided using a vertical or horizontal predictor to replacea JPEG-LS predictor. The vertical or horizontal predictor may be chosenand signaled at a block level. The shape of the independently decodableregions reflects the geometry of the predictor. Due to the shape of thehorizontal or vertical predictors, which use a left or a top pixel forprediction of the current pixel, the most throughput-efficient way ofprocessing the block may be to process all the pixels of one column orrow in parallel, and to process these columns or rows sequentially. Forexample, in order to increase throughput, a block of width 4 is dividedinto two halves with a horizontal boundary when the predictor chosen onthis block is vertical, and a block of height 4 is divided into twohalves with a vertical boundary when the predictor chosen on this blockis horizontal. For a 4×4 block, an 8×4, or a 4×8 block with twoindependently decodable areas, the throughput may be 8 pixels per cycle.For a 4×8 block, an 8×4 block, or an 8×8 block without independentlydecodable areas, the throughput may be 8 pixels per cycle. For a 16×16block without independently decodable areas, the throughput may be 16pixels per cycle.

In Test 3, according to an embodiment of the present disclosure, theBDPCM residue amplitude is limited to 28, and the amplitude is encodedwith truncated unary binarization for the first 12 bins, followed byorder-2 Exp-Golomb equal probability bins for the remainder (e.g., usingan encodeRemAbsEP( )function).

IV. Transform Coefficient Coding

Entropy coding can be performed at a last stage of video coding (or afirst stage of video decoding) after a video signal is reduced to aseries of syntax elements. Entropy coding can be a lossless compressionscheme that uses statistic properties to compress data such that anumber of bits used to represent the data are logarithmicallyproportional to the probability of the data. For example, by performingentropy coding over a set of syntax elements, bits representing thesyntax elements (referred to as bins) can be converted to fewer bits(referred to as coded bits) in a bit stream. CABAC is one form ofentropy coding. In CABAC, a context model providing a probabilityestimate can be determined for each bin in a sequence of bins based on acontext associated with the respective bin. Subsequently, a binaryarithmetic coding process can be performed using the probabilityestimates to encode the sequence of bins to coded bits in a bit stream.In addition, the context model is updated with a new probabilityestimate based on the coded bin.

FIG. 9A shows an exemplary CABAC based entropy encoder (900A) inaccordance with an embodiment. For example, the entropy encoder (900A)can be implemented in the entropy coder (545) in the FIG. 5 example, orthe entropy encoder (625) in the FIG. 6 example. The entropy encoder(900A) can include a context modeler (910) and a binary arithmeticencoder (920). In an example, various types of syntax elements areprovided as input to the entropy encoder (900A). For example, a bin of abinary valued syntax element can be directly input to the contextmodeler (910), while a non-binary valued syntax element can be binarizedto a bin string before bins of the bin string are input to the contextmodeler (910).

In an example, the context modeler (910) receives bins of syntaxelements, and performs a context modeling process to select a contextmodel for each received bin. For example, a bin of a binary syntaxelement of a transform coefficient in a transform block is received. Thetransform block may be a transform skipped block when the current blockis coded with BDPCM for prediction. A context model can accordingly bedetermined for this bin based, for example, on a type of the syntaxelement, a color component type of the transform component, a locationof the transform coefficient, and previously processed neighboringtransform coefficients, and the like. The context model can provide aprobability estimate for this bin.

In an example, a set of context models can be configured for one or moretypes of syntax elements. Those context models can be arranged in acontext model list (902) that is stored in a memory (901) as shown inFIG. 9A. Each entry in the context model list (902) can represent acontext model. Each context model in the list can be assigned an index,referred to as a context model index, or context index. In addition,each context model can include a probability estimate, or parametersindicating a probability estimate. The probability estimate can indicatea likelihood of a bin being 0 or 1. For example, during the contextmodeling, the context modeler (910) can calculate a context index for abin, and a context model can accordingly be selected according to thecontext index from the context model list (902) and assigned to the bin.

Moreover, probability estimates in the context model list can beinitialized at the start of the operation of the entropy encoder (900A).After a context model in the context model list (902) is assigned to abin and used for encoding the bin, the context model can subsequently beupdated according to a value of the bin with an updated probabilityestimate.

In an example, the binary arithmetic encoder (920) receives bins andcontext models (e.g., probability estimates) assigned to the bins, andaccordingly performs a binary arithmetic coding process. As a result,coded bits are generated and transmitted in a bit stream.

FIG. 9B shows an exemplary CABAC based entropy decoder (900B) inaccordance with an embodiment. For example, the entropy decoder (900B)can be implemented in the parser (420) in the FIG. 4 example, or theentropy decoder (771) in the FIG. 7 example. The entropy decoder (900B)can include a binary arithmetic decoder (930), and a context modeler(940). The binary arithmetic decoder (930) receives coded bits from abit stream, and performs a binary arithmetic decoding process to recoverbins from the coded bits. The context modeler (940) can operatesimilarly to the context modeler (910). For example, the context modeler(940) can select context models in a context model list (904) stored ina memory (903), and provide the selected context models to the binaryarithmetic decoder (930). The context modeler (940) can determine thecontext models based on the recovered bins from the binary arithmeticdecoder (930). For example, based on the recovered bins, the contextmodeler (940) can know a type of a syntax element of a nextto-be-decoded bin, and values of previously decoded syntax elements.That information is used for determining a context model for the nextto-be-decoded bin.

V. Entropy Coding for Transform Coefficients

1. Syntax Elements of Transform Coefficients

In an embodiment, residual signals of a transform block are firsttransformed from spatial domain to frequency domain resulting in a blockof transform coefficients. Then, a quantization is performed to quantizethe block of transform coefficients into a block of transformcoefficient levels. In various embodiments, different techniques may beused for converting residual signals into transform coefficient levels.The block of transform coefficient levels is further processed togenerate syntax elements that can be provided to an entropy encoder andencoded into bits of a bit stream. In an embodiment, a process ofgenerating the syntax elements from the transform coefficient levels canbe performed in the following way.

The block of transform coefficient levels can be first split intosub-blocks, for example, with a size of 4×4 positions. Those sub-blockscan be processed according to a predefined scan order. FIG. 10 shows anexample of the sub-block scan order, referred to as an inverse diagonalscan order. As shown, a block (1010) is partitioned into sixteensub-blocks (1001). Each sub-block (1001) may be a coefficient group(CG). Before a position in the sub-block (1001) is processed or scanned,a flag may be signaled to indicate whether the CG includes at least onenon-zero transform coefficient level. When the flag indicates the CGincludes at least one non-zero transform coefficient level, thesub-block at the bottom-right corner is first processed, and thesub-block at the top-left corner is last processed. For a sub-blockwithin which the transform coefficient levels are all zero, thesub-block can be skipped without processing in an example. In a TS mode,the scan order may be the opposite of the inverse diagonal scan order.That is, the sub-block at the top-left corner may be first processed,and the sub-block at the bottom-right corner may be last processed.

For sub-blocks each having at least one non-zero transform coefficientlevel, four passes of a scan can be performed in each sub-block. Duringeach pass, the 16 positions in the respective sub-block can be scannedin the inverse diagonal scan order. FIG. 11 shows an example of asub-block scanning process (1100) from which different types of syntaxelements of transform coefficients can be generated.

Sixteen coefficient positions (1110) inside a sub-block are shown in onedimension at the bottom of FIG. 11. The positions (1110) are numberedfrom 0 to 15 reflecting the respective scan order. During a first pass,the scan positions (1110) are scanned over, and three types of syntaxelements (1101-1103) may be generated at each scan position (1110) asfollows:

(i) A first type of binary syntax elements (1101) (referred to assignificance flags and denoted by sig_coeff_flag) indicating whether anabsolute transform coefficient level (hereinafter referred to as“absLevel”) of the respective transform coefficient is zero or largerthan zero. The absolute transform coefficient level generally refers tothe magnitude of the transform coefficient value. The absolute transformcoefficient level combined with a sign value (positive or negative) canrepresent the transform coefficient.

(ii) A second type of binary syntax elements (1102) (referred to asparity flags and denoted by par_level_flag) indicating a parity of theabsolute transform coefficient level of the respective transformcoefficient. The parity flags are generated only when the absolutetransform coefficient level of the respective transform coefficient isnon-zero.

(iii) A third type of binary syntax elements (1103) (referred to asgreater than 1 flags and denoted by rem_abs_gt1_flag) indicating whether(absLevel−1)>>1 is greater than 0 for the respective transformcoefficient. The greater than 1 flags are generated only when theabsolute transform coefficient level of the respective transformcoefficient is non-zero.

During a second pass, a fourth type of binary syntax elements (1104) maybe generated. The fourth type of syntax elements (1104) is referred toas greater than 2 flags and denoted by rem_abs_gt2_flag. The fourth typeof syntax elements (1104) indicates whether the absolute transformcoefficient level of the respective transform coefficient is greaterthan 4. The greater than 2 flags are generated only when (absLevel−1)>>1is greater than 0 for the respective transform coefficient.

During a third pass, a fifth type of non-binary syntax elements (1105)can possibly be generated. A remainder generally refers to a remainingvalue of the absolute transform coefficient level absLevel (e.g.,abs_remainder). In coefficient coding, at least a first signal isgenerated to indicate whether the coefficient level is greater than apredetermined value X. Subsequently, a second signal is generatedcorresponding to the remainder value of the absolute transformcoefficient level (e.g., absLevel-X). For example, here, the fifth typeof syntax elements (1105) may be denoted by abs_remainder, and indicatesa remaining value of the absolute transform coefficient level of therespective transform coefficient that is greater than 4. The fifth typeof syntax elements (1105) are generated only when the absolute transformcoefficient level of the respective transform coefficient is greaterthan 4.

During a fourth pass, a sixth type of syntax elements (1106) can begenerated at each scan position (1110) with a non-zero coefficient levelindicating a sign of the respective transform coefficient level.

In a TS mode, the first pass may include the significance flags, theparity flags, the greater than 1 flags, and the greater than 2 flags.Moreover, additional types of syntax elements such as greater than xflags may be generated during a separate pass. The greater than x flagsmay be denoted by rem_abs_gtx_flag and indicate whether the absolutetransform coefficient level of the respective transform coefficient isgreater than x. In some examples, x can be 2, 4, 6, or 8.

The above described various types of syntax elements can be provided toan entropy encoder according to the order of the passes and the scanorder in each pass. Different entropy encoding schemes can be employedfor encoding different types of syntax elements. For example, in anembodiment, the significance flags, parity flags, greater than 1 flags,and greater than 2 flags can be encoded with a CABAC based entropyencoder, as described in FIG. 9A . In contrast, the syntax elementsgenerated during the third and fourth passes can be encoded with aCABAC-bypassed entropy encoder (e.g., a binary arithmetic encoder withfixed probability estimates for input bins).

2. Context Modeling of Bins of Transform Coefficient Syntax Elements

Context modeling can be performed to determine context models for binsof some types of transform coefficient syntax elements. In anembodiment, in order to exploit the correlation among the transformcoefficients, the context models can be determined according to a localtemplate and a diagonal position of each current coefficient (e.g., acoefficient currently under processing) possibly in combination withother factors.

FIG. 12 shows an example of a local template (1230) used for contextselection for current coefficients. The local template (1230) can covera set of neighboring positions or coefficients of a current coefficient(1220) in a coefficient block (1210). The coefficient block (1210) canhave a size of 8×8 positions, and include coefficient levels at the 64positions. The coefficient block (1210) is partitioned into 4 sub-blockseach with a size of 4×4 positions. Each sub-block may be a CG that caninclude 4×4 coefficient positions. The CG (1230) includes the currentcoefficient (1210). A flag may be signaled to indicate whether the CG(1230) includes only zero coefficient levels. In the FIG. 12 example,the local template (1230) is defined to be a 5 position templatecovering 5 coefficient levels at the bottom-right side of the currentcoefficient (1220). When an inverse diagonal scan order is used formultiple passes over the scan positions within the coefficient block(1210), the neighboring positions within the local template (1230) areprocessed prior to the current coefficient (1220). In a TS mode, thescan order may be the opposite of the inverse diagonal scan order andthe local template may be a 5 position template cover 5 coefficientlevels at the top-left side of the current coefficient.

During the context modeling, information of the coefficient levelswithin the local template (1230) can be used to determine a contextmodel. For this purpose, a measure, referred to as a template magnitude,is defined in some embodiments to measure or indicate magnitudes of thetransform coefficients or transform coefficient levels within the localtemplate (1230). The template magnitude can then be used as the basisfor selecting the context model.

In one example, the template magnitude is defined to be a sum, denotedby sumAbs1, of partially reconstructed absolute transform coefficientlevels inside the local template (1230). A partially reconstructedabsolute transform coefficient level can be determined according to binsof the syntax elements, sig_coeff_flag, par_level_flag, andrem_abs_gt1_flag of the respective transform coefficient. Those threetypes of syntax elements may be obtained after a first pass over scanpositions of a sub-block performed in an entropy encoder or an entropydecoder. In an embodiment, numSig is a number of non-zero coefficientsin the local template (1230). Moreover, a diagonal position of a scanposition (x, y) d is defined according to: d=×+y, where x and y arecoordinates of the respective position. The context model index may beselected based on sumAbs1 and the diagonal position d as describedbelow.

3. Context Index Determination Based on Local Template and DiagonalPosition

In an embodiment, during a context modeling process in an entropyencoder or decoder, context indices can be determined as described belowfor the context coded binary syntax elements of the current coefficient(1220). The determination can be based on the local template (1230) andthe diagonal position of the current coefficient (1220).

(1) sig_coeff_flag

When coding sig_coeff_flag of the current coefficient (1220), thecontext index can be selected depending on sumAbs1 and diagonal positiond of the current coefficient (1220). For example, for luma component,the context index is determined according to:

ctxSig=18*max(0, state−1)+min(sumAbs1,5)+(d<2?12:(d<5?6:0))   (Eq. 7)

where ctxSig represents the context index of the significance flagsyntax element, and “state” specifies a state of a scaler quantizer of adependent quantization scheme where the state can have a value of 0, 1,2, or 3.

The Eq. 7 is equivalent to the following formulas:

ctxIdBase=18*max(0, state−1)+(d<2?12:(d<5?6:0)),   (Eq. 8)

ctxSig=ctxIdSigTable[min(sumAbs1,5)]+ctxIdBase.   (Eq. 9)

In Eq. 8 and Eq. 9, ctxIdBase represents a context index base. Thecontext index base can be determined based on the state and the diagonalposition d. For example, the state can have a value of 0, 1, 2, or 3,and accordingly max(0, state-1) can have one of three possible values,0, 1, or 2. For example, (d<2? 12: (d<5? 6: 0)) can take a value of 12,6, or 0, corresponding to different ranges of d: d<2, 2<=d<5, or 5<=d.

In Eq. 8 and Eq. 9, ctxIdSigTable[ ] can represent an array datastructure, and can store context index offsets of significance flagswith respect to the ctxIdBase. For example, for different sumAbs1values, min(sumAbs1, 5) clips a sumAbs1 value to be smaller than orequal to 5. Then, the clipped value is mapped to a context index offset.For example, under the definition of ctxIdSigTable[0˜5]={0, 1, 2, 3, 4,5}, the clipped value 0, 1, 2, 3, 4, or 5 are mapped to 0, 1, 2, 3, 4,or 5, respectively.

For chroma component, the context index can be determined according to:

ctxSig=12*max(0, state−1)+min(sumAbs1, 5)+(d<2?6:0),   (Eq. 10)

which is equivalent to the following formulas:

ctxIdBase=12*max(0, state−1)+(d<2?6:0),   (Eq. 11)

ctxSig=ctxIdSigTable[min(sumAbs1, 5)]+ctxIdBase.   (Eq. 12)

(2) par_level_flag

When coding par_level_flag of the current coefficient (1220), thecontext index can be selected depending on sumAbs1, numSig, and diagonalposition d. For example, for luma component, if the current coefficientis the first non-zero coefficient in decoding order, the context indexctxPar is assigned to be 0; otherwise the context index can bedetermined according to:

ctxPar=1+min(sumAbs1−numSig, 4)+(d==0?15:(d<3?10:(d<10?5:0))),   (Eq.13)

which is equivalent to the following formulas:

ctxIdBase=(d==0?15:(d<3?10:(d<10?5:0 ))),   (Eq. 14)

ctxPar=1+ctxIdTable[min(sumAbs1−numSig, 4)]+ctxIdBase,   (Eq. 15)

where ctxPar represents the context index of the parity flag, andctxIdTable[ ] represents another array data structure, and storescontext index offsets with respect to the respective ctxIdBase. Forexample, ctxIdTable[0˜4]={0, 1, 2, 3, 4}.

For chroma, if the current coefficient is the first non-zero coefficientin decoding order, the context index ctxPar is assigned to be 0;otherwise, the context index can be determined according to:

ctxPar=1+min(sumAbs1−numSig, 4)+(d==0?5:0),   (Eq. 16)

which is equivalent to the following formulas:

ctxIdBase=(d==0?5:0),   (Eq. 17)

ctxPar=1+ctxIdTable[min(sumAbs1−numSig, 4)]+ctxIdBase.   (Eq. 18)

(3) rem_abs_gt1_flag and rem_abs_gt2_flag

When coding rem_abs_gt1_flag and rem_abs_gt2_flag of the currentcoefficient (1220), the context model indices can be determined in thesame way as par_level_flag:

ctxGt1=ctxPar,

ctxGt2=ctxPar,

where ctxGt1 and ctxGt2 represent the context indices of the greaterthan 1 and greater than 2 flags, respectively.

It is noted that different sets of context models are used for differenttypes of the syntax elements, sig_coeff_flag, par_level_flag,rem_abs_gt1_flag and rem_abs_gt2_flag. For example, the context modelused for rem_abs_gt1_flag is different from that of rem_abs_gt2_flag,even though a value of ctxGt1 is equal to that of ctxGt2.

VI. Improved Residual Coding for the TS mode and the BDPCM mode

The residual coding for both the TS mode and the BDPCM mode is processedin the spatial domain without transform. Therefore, a shared modulebetween the TS mode and the BDPCM mode can be used for a simpler designof coefficient coding.

Moreover, the coefficients of the TS mode and the BDPCM mode showdifferent characteristics from regular transform coefficients which areassociated with transform and quantization. Therefore, a differentcoefficient coding scheme may show better coding performance.

In an embodiment, for coefficient coding of the TS mode and/or the BDPCMmode, the parity bit flag (e.g., par_level-flag) which indicates theparity of the absolute transform coefficient level (e.g., absLevel) maybe bypass coded without using context modeling in arithmetic coding. Inan example, the parity bit flag can be coded in a separate pass. Inanother example, the parity bit flag can be coded together withabs_remainder in a same pass.

In an embodiment, for coefficient coding of the TS mode and/or the BDPCMmode, the parity bit flag (e.g., par_level_flag) which indicates theparity of the absolute transform coefficient level (e.g., absLevel) maybe coded in a separate pass or coded together with other syntax elementsin one pass with a certain order or combination. In an example, theparity bit flag can be coded in a separate pass after the coding passwhich codes sig_coeff_flag and rem_abs_gt1_flag. In another example, theparity bit flag can be coded in a separate pass after the coding passwhich codes rem_abs_gt2_flag or a greater than x flag (e.g.,rem_abs_gtx_flag), where x can be 2, 4, 6, or 8. In an example, theparity bit flag can be coded together with rem_abs_gt2_flag or thegreater than x flag in a same coding pass. In an example, the parity bitflag can be coded together with sign information in a same coding pass.

In an embodiment, for coefficient coding of the TS mode and/or the BDPCMmode, when coding the syntax element rem_abs_gt2_flag, the number ofcontext coded bins per coefficient for coding rem_abs_gt2_flag maydepend on coded information. The coded information may include thenumber of nonzero coefficients (e.g., number of coded sig_coeff_flag inthe current CG), the number of nonzero coefficients (e.g., number ofcoded sig_coeff_flag) in the previously coded CG, whether the currentblock is coded by intra prediction or inter prediction or IBC mode, thecurrent coefficient block size, coefficient block width, coefficientblock height, and coefficient block aspect ratio. In some examples, thecurrent CG includes the coefficients.

In an example, if a current block is intra coded, the number of contextcoded bins per coefficient for coding rem_abs_gt2_flag may be more thanthe number of context coded bins per coefficient for codingrem_abs_gt2_flag when current block is inter coded and/or IBC coded.Exemplary values of the number of context coded bins per coefficient forcoding rem_abs_gt2flag when current block is inter coded include 3, 4,5, and 6. Exemplary values of the number of context coded bins percoefficient for coding rem_abs_gt2_flag when current block is intracoded include 5, 6, 7, 8, and 9.

In an embodiment, for coefficient coding of the TS mode and/or the BDPCMmode, the maximum number of context coded bins of the syntax elementsper CG or the maximum average number of context coded bins of the syntaxelements per CG depends on coded information. The coded informationincludes the number of nonzero coefficients (e.g., number of codedsig_coeff_flag in the current CG), the number of nonzero coefficients(e.g., number of coded sig_coeff_flag) in the previously coded CG,whether the current block is coded by intra prediction or interprediction or IBC mode, the current coefficient block size, coefficientblock width, coefficient block height, and coefficient block aspectratio. Setting a maximum number of context coded bins not only booststhe coding speed, but also reduces required memory size and cost ofmaintaining the context models.

In an embodiment, for coefficient coding of the TS mode and/or the BDPCMmode, the parity bit flag (e.g., par_level_flag) which indicates theparity of absLevel may not be coded. Instead, the syntax elementssig_coeff_flag, abs_gt1_flag, abs_gt2_flag, the sign information, andabs_remainder are coded.

In an example, the syntax elements sig_coeff_flag and abs_gt1_flag maybe coded in the first pass. The syntax element abs_gt2_flag may be codedin the second pass. The syntax element abs_remainder may be coded in thethird pass. If necessary, the sign information may be coded in thefourth pass.

In another example, the syntax elements sig_coeff_flag, abs_gt1_flag,and sign information may be coded in the first pass. The syntax elementabs_gt2_flag may be coded in the second pass. The syntax elementabs_remainder may be coded in the fourth pass.

In an embodiment, for coefficient coding of the TS mode and/or the BDPCMmode, in addition to the syntax elements rem_abs_gt1_flag (e.g., whichindicates absLevel is greater than 1), and rem_abs_gt2_flag (e.g., whichindicates absLevel is greater than 3), additional syntax elementsrem_abs_gt3_flag (e.g., which indicates absLevel is greater than 5)and/or rem_abs_gt4_flag (e.g., which indicates absLevel is greater than7) may be also signaled. The syntax elements rem_abs_gt3_flag and/orrem_abs_gt4_flag may use separate contexts for entropy coding.

In an embodiment, for coefficient coding of the TS mode and/or the BDPCMmode, when coding the sign information for each non-zero coefficient,the context may depend on previously coded sign bit values.

In an example, FIG. 13A shows that the context used for coding the signinformation includes the above block (1301) and the left block (1302) ofthe current coefficient (marked as X). In an example, FIG. 13B showsthat the context used for coding the sign information includes multipleabove and left blocks (1303, 1304, 1305, 1306, and 1307) of the currentcoefficient (marked as X).

In an embodiment, the context used for coding the sign informationdepends on the previously coded N sign bits. Exemplary values of Ninclude 1, 2, 3, and 4.

In an embodiment, instead of coding the sign information, a signresidual is coded, and the sign residual bit indicates whether thecurrent sign bit is equal to a predicted sign bit value. In an example,the predicted sign bit is derived using previous scanned N sign bits.Exemplary values of N include 1, 2, 3, and 4. In another example, thepredicted sign bit is derived using left and/or top neighboring sign bitvalues.

In an embodiment, for coefficient coding of the TS mode and/or the BDPCMmode, when coding the magnitude of coefficients, one or more primarylevel values may be first signaled, then the residual of each non-zerocoefficient minus one of these primary level values are signaled. In anembodiment, the primary level values are limited to a given threshold.In an embodiment, the number of primary level values is limited to agiven threshold, such as 1, 2, 3, or 4.

In an embodiment, when multiple primary level values are signaled, thenthe primary level values are arranged in an ascending order, then foreach coefficient, starting from the smallest primary level value, a flagindicating whether the current coefficient has a level value greaterthan the current primary level value is signaled. In an example, whenthe current coefficient has a level value which is not greater than thecurrent primary level value, then the difference between the currentlevel value and the previous primary level value is signaled.

VII. Exemplary Decoding Processes

FIG. 14 shows a flow chart outlining a coefficient decoding process(1400) according to some embodiments of the disclosure. The process(1400) can be used in entropy decoding of several types of coefficientsyntax elements. In various embodiments, the process (1400) can beexecuted by processing circuitry, such as the processing circuitry inthe terminal devices (210), (220), (230) and (240), the processingcircuitry that performs functions of the video decoder (310), theprocessing circuitry that performs functions of the video decoder (410),and the like. In some embodiments, the process (1400) is implemented insoftware instructions, thus when the processing circuitry executes thesoftware instructions, the processing circuitry performs the process(1400). The process starts at (S1401) and proceeds to (S1410).

At (S1410), a bit stream including coded bits of bins of syntax elementsis received. The syntax elements correspond to coefficients of a regionof a transform skipped block in a coded picture, and include a firstflag and a second flag. The first flag indicates whether an absolutecoefficient level of one of the coefficients is greater than a firstthreshold (e.g., 1), and the second flag indicates a parity of theabsolute coefficient level. For example, the first flag may be asignificance syntax element (e.g., sig_coeff_flag) that indicates thatan absolute value of the current coefficient (e.g., absLevel) is greaterthan the first threshold. The second flag may be a parity syntax element(e.g., par_level_flag) that indicates the parity of absLevel. Thetransform skipped block may indicate that a transform was not performedon the transform block. For example, when the current block is codedwith BDPCM, a transform is not performed on the transform block.

At (S1420), the second flag is decoded in a separate pass. For example,the pass satisfies at least one of: (1) no other syntax elements isdecoded in the pass, (2) a third flag indicating whether the absolutecoefficient level is greater than a second threshold (e.g., 3) isdecoded in the pass, and (3) a fourth flag indicating sign informationof the coefficient level of the one of the coefficients is decoded inthe pass. The second threshold is greater than the first threshold insome embodiments. In an example, the second flag, which is a paritysyntax element, may be decoded in a separate pass after the first flagand a fifth flag are decoded. The fifth flag indicates whether theabsolute coefficient level is greater than a third threshold (e.g., 3).In an example, the second flag is decoded in a separate pass after thethird flag is decoded in a previous pass. In an example, the second flagis decoded in the same pass with the third flag or the fourth flag. Theprocess (1400) proceeds to and terminates at (S1499).

FIG. 15 shows a flow chart outlining a coefficient decoding process(1500) according to some embodiments of the disclosure. The process(1500) can be used in entropy decoding of several types of coefficientsyntax elements. In various embodiments, the process (1500) can beexecuted by processing circuitry, such as the processing circuitry inthe terminal devices (210), (220), (230) and (240), the processingcircuitry that performs functions of the video decoder (310), theprocessing circuitry that performs functions of the video decoder (410),and the like. In some embodiments, the process (1500) is implemented insoftware instructions, thus when the processing circuitry executes thesoftware instructions, the processing circuitry performs the process(1500). The process starts at (S1501) and proceeds to (S1510).

At (S1510), a bit stream including coded bits of bins of syntax elementsis received. The syntax elements correspond to coefficients of a regionof a transform skipped block in a coded picture, and include a firstflag and a second flag. The first flag indicates whether an absolutecoefficient level of one of the coefficients is greater than a firstthreshold (e.g., 1), and the second flag indicates whether the absolutecoefficient level of the one of the coefficients is greater than asecond threshold (e.g., 3). The second threshold is greater than thefirst threshold in some embodiments. For example, the first flag may bea significance syntax element (e.g., sig_coeff_flag) that indicates thatan absolute value of the current coefficient (e.g., absLevel) is greaterthan the first threshold. The second flag may be a greater than 2 syntaxelement (e.g., rem_abs_gt2_flag), which indicates whether the absolutetransform coefficient level of the respective transform coefficient isgreater than the second threshold. The transform skipped block mayindicate that a transform was not performed on the transform block. Forexample, when the current block is coded with BDPCM, a transform is notperformed on the transform block.

At (S1520), a number of the bins of the second flag that are contextcoded is determined based on coded information of the coefficients. Forexample, the coded information of the coefficients includes a number ofthe first flag in a current coefficient group (CG) including thecoefficients, a number the first flag in a previous CG, whether acurrent block corresponding to the transform skipped block is intracoded or inter coded, a size of the transform skipped block, a width ofthe transformed skipped block, a height of the transformed skippedblock, or an aspect ratio of the transformed skipped block. The previousCG may be a neighboring CG that has been scanned. In one example, whenthe current block is intra coded, the number of the bins of the secondflag is more than the number of the bins of the second flag when thecurrent block is inter coded.

At (S1530), context modeling is performed to determine a context modelfor each of the number of the bins of the second flag. The number of thebins of the second flag that are context coded may not exceed the numberdetermined at S1520.

At (S1540), the coded bits of the number of the bins of the second flagare decoded based on the determined context models. The coded bits ofthe remaining total number of the bins of the second flag for the regionof the transform skipped block may be decoded based on an EP model(i.e., a bypass model). Based on the recovered bins, coefficient levelsof the coefficients can be reconstructed. The process (1500) proceeds toand terminates at (S1599).

FIG. 16 shows a flow chart outlining a coefficient decoding process(1600) according to some embodiments of the disclosure. The process(1600) can be used in entropy decoding of several types of coefficientsyntax elements. In various embodiments, the process (1600) can beexecuted by processing circuitry, such as the processing circuitry inthe terminal devices (210), (220), (230) and (240), the processingcircuitry that performs functions of the video decoder (310), theprocessing circuitry that performs functions of the video decoder (410),and the like. In some embodiments, the process (1600) is implemented insoftware instructions, thus when the processing circuitry executes thesoftware instructions, the processing circuitry performs the process(1600). The process starts at (S1601) and proceeds to (S1610).

At (S1610), a bit stream including coded bits of bins of syntax elementsis received. The syntax elements correspond to coefficients included ina coefficient group (CG) of a transform skipped block in a codedpicture. The CG may be partitioned by a transform skippedblock/coefficient block and may include 4×4 coefficient positions. Thetransform skipped block may indicate that a transform was not performedon the transform block. For example, when the current block is codedwith BDPCM, a transform is not performed on the transform block.

At (S1620), a maximum number of context coded bins of the CG or amaximum average number of context coded bins of the CG is determinedbased on coded information of the coefficients. For example, the codedinformation of the coefficients includes a number of the first flag inthe CG including the coefficients, a number the first flag in a previousCG, whether a current block corresponding to the transform skipped blockis intra coded or inter coded, a size of the transform skipped block, awidth of the transformed skipped block, a height of the transformedskipped block, or an aspect ratio of the transformed skipped block. Theprevious CG may be a neighboring CG that has been scanned.

At (S1630), context modeling is performed to determine a context modelfor each of a number of the bins of syntax elements in the CG. Thenumber of the bins of syntax elements in the CG being context coded maynot exceed the maximum number of context coded bins of the CG or themaximum average number of context coded bins of the CG determined atS1620.

At (S1640), the coded bits of the number of the bins of syntax elementsare decoded based on the determined context models. The coded bits ofthe remaining total number of the bins of syntax elements in the CG maybe decoded based on an EP model (i.e., a bypass model). Based on therecovered bins, coefficient levels of the coefficients can bereconstructed. The process (1600) proceeds to and terminates at (S1699).

VIII. Computer System

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 17 shows a computersystem (1700) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 17 for computer system (1700) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1700).

Computer system (1700) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1701), mouse (1702), trackpad (1703), touchscreen (1710), data-glove (not shown), joystick (1705), microphone(1706), scanner (1707), camera (1708).

Computer system (1700) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1710), data-glove (not shown), or joystick (1705), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1709), headphones(not depicted)), visual output devices (such as screens (1710) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1700) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1720) with CD/DVD or the like media (1721), thumb-drive (1722),removable hard drive or solid state drive (1723), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1700) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (1749) (such as, for example USB ports of thecomputer system (1700)); others are commonly integrated into the core ofthe computer system (1700) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1700) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1740) of thecomputer system (1700).

The core (1740) can include one or more Central Processing Units (CPU)(1741), Graphics Processing Units (GPU) (1742), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1743), hardware accelerators for certain tasks (1744), and so forth.These devices, along with Read-only memory (ROM) (1745), Random-accessmemory (1746), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (1747), may be connectedthrough a system bus (1748). In some computer systems, the system bus(1748) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (1748),or through a peripheral bus (1749). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1741), GPUs (1742), FPGAs (1743), and accelerators (1744) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1745) or RAM (1746). Transitional data can be also be stored in RAM(1746), whereas permanent data can be stored for example, in theinternal mass storage (1747). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1741), GPU (1742), massstorage (1747), ROM (1745), RAM (1746), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1700), and specifically the core (1740) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1740) that are of non-transitorynature, such as core-internal mass storage (1747) or ROM (1745). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1740). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1740) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1746) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1744)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

Appendix A: Acronyms

JEM: joint exploration modelVVC: versatile video codingBMS: benchmark set

MV: Motion Vector HEVC: High Efficiency Video Coding SEI: SupplementaryEnhancement Information VUI: Video Usability Information GOPs: Groups ofPictures TUs: Transform Units, PUs: Prediction Units CTUs: Coding TreeUnits CTBs: Coding Tree Blocks PBs: Prediction Blocks HRD: HypotheticalReference Decoder SNR: Signal Noise Ratio CPUs: Central Processing UnitsGPUs: Graphics Processing Units CRT: Cathode Ray Tube LCD:Liquid-Crystal Display OLED: Organic Light-Emitting Diode CD: CompactDisc DVD: Digital Video Disc ROM: Read-Only Memory RAM: Random AccessMemory ASIC: Application-Specific Integrated Circuit PLD: ProgrammableLogic Device LAN: Local Area Network

GSM: Global System for Mobile communications

LTE: Long-Term Evolution CANBus: Controller Area Network Bus USB:Universal Serial Bus PCI: Peripheral Component Interconnect FPGA: FieldProgrammable Gate Areas

SSD: solid-state drive

IC: Integrated Circuit CU: Coding Unit DPCM: Differential Pulse-codeModulation BDPCM: Block Differential Pulse-code Modulation SCC: ScreenContent Coding

Bs: Boundary Strength

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

What is claimed is:
 1. A method of video decoding performed in a videodecoder, the method comprising: receiving a bit stream including bins ofsyntax elements, the syntax elements corresponding to coefficients of aregion of a transform skipped block in a coded picture, the syntaxelements including a first flag indicating whether an absolutecoefficient level of one of the coefficients is greater than a firstthreshold value, and a second flag indicating a parity of the absolutecoefficient level; and decoding the second flag in a pass, wherein thepass satisfies at least one of: (1) no other syntax elements is decodedin the pass; (2) a third flag indicating whether the absolutecoefficient level is greater than a second threshold value is decoded inthe pass; and (3) a fourth flag indicating sign information of thecoefficient level of the one of the coefficients is decoded in the pass,the second threshold value being greater than the first threshold value.2. The method of claim 1, wherein the first threshold value is 1 and thesecond threshold value is
 3. 3. The method of claim 1, wherein a currentblock corresponding to the transform skipped block is coded with a blockdifferential pulse-code modulation mode.
 4. The method of claim 1,wherein the decoding further comprises: decoding the second flag in thepass without decoding other syntax elements after decoding the firstflag and a fifth flag indicating whether the absolute coefficient levelis greater than 3 in a previous pass.
 5. The method of claim 1, whereinthe decoding further comprises: decoding the second flag in the passwithout decoding other syntax elements after decoding the third flag ina previous pass.
 6. The method of claim 1, wherein the decoding furthercomprises: decoding the second flag in the same pass with the thirdflag.
 7. The method of claim 1, wherein the decoding further comprises:decoding the second flag in the same pass with the fourth flag.
 8. Amethod of video decoding performed in a video decoder, the methodcomprising: receiving a bit stream including bins of syntax elements,the syntax elements corresponding to coefficients of a region of atransform skipped block in a coded picture, the syntax elementsincluding a first flag indicating whether an absolute coefficient levelof one of the coefficients is greater than a first threshold value, anda second flag indicating whether the absolute coefficient level of theone of the coefficients is greater than a second threshold value, thesecond threshold value being greater than the first threshold value;determining a number of the bins of the second flag that are contextcoded based on coded information of the coefficients; performing contextmodeling to determine a context model for each of the number of the binsof the second flag; and decoding the number of the bins of the secondflag based on the determined context models.
 9. The method of claim 8,wherein the first threshold value is 1 and the second threshold value is3.
 10. The method of claim 8, wherein the coded information of thecoefficients includes a number of the first flag in a currentcoefficient group (CG) including the coefficients, a number the firstflag in a previous CG, whether a current block corresponding to thetransform skipped block is intra coded or inter coded, a size of thetransform skipped block, a width of the transformed skipped block, aheight of the transformed skipped block, or an aspect ratio of thetransformed skipped block.
 11. The method of claim 10, wherein when thecurrent block is intra coded, the number of the bins of the second flagis more than the number of the bins of the second flag when the currentblock is inter coded.
 12. A method of video decoding performed in avideo decoder, the method comprising: receiving a bit stream includingbins of syntax elements, the syntax elements corresponding tocoefficients included in a coefficient group (CG) of a transform skippedblock in a coded picture; determining a maximum number of context codedbins of the CG or a maximum average number of context coded bins of theCG based on coded information of the coefficients; performing contextmodeling to determine a context model for each of a number of the binsof syntax elements in the CG, the number of the bins of syntax elementsin the CG being context coded not exceeding the maximum number ofcontext coded bins of the CG or the maximum average number of contextcoded bins of the CG; and decoding the number of the bins of syntaxelements based on the determined context models.
 13. The method of claim10, wherein the coded information of the coefficients includes a numberof the first flag in the CG including the coefficients, a number thefirst flag in a previous CG, whether a current block corresponding tothe transform skipped block is intra coded or inter coded, a size of thetransform skipped block, a width of the transformed skipped block, aheight of the transformed skipped block, or an aspect ratio of thetransformed skipped block.
 14. An apparatus of video decoding,comprising: processing circuitry configured to: receive a bit streamincluding bins of syntax elements, the syntax elements corresponding tocoefficients of a region of a transform skipped block in a codedpicture, the syntax elements including a first flag indicating whetheran absolute coefficient level of one of the coefficients is greater thana first threshold, and a second flag indicating a parity of the absolutecoefficient level; and decode the second flag in a pass, wherein thepass satisfies at least one of: (1) no other syntax elements is decodedin the pass; (2) a third flag indicating whether the absolutecoefficient level is greater than a second threshold is decoded in thepass; and (3) a fourth flag indicating sign information of thecoefficient level of the one of the coefficients is decoded in the pass,the second threshold being greater than the first threshold.
 15. Theapparatus of claim 14, wherein a current block corresponding to thetransform skipped block is coded with a block differential pulse-codemodulation mode.
 16. The apparatus of claim 14, wherein the processingcircuitry is further configured to decode the second flag in the passwithout other syntax elements after decoding the first flag and a fifthflag indicating whether the absolute coefficient level is greater than 3in a previous pass.
 17. The apparatus of claim 14, wherein theprocessing circuitry is further configured to decode the second flag inthe separate pass after decoding the third flag in a previous pass. 18.The apparatus of claim 14, wherein the processing circuitry is furtherconfigured to decode the second flag in the same pass with the thirdflag.
 19. The apparatus of claim 14, wherein the processing circuitry isfurther configured to decode the second flag in the same pass with thefourth flag.
 20. An apparatus of video decoding, comprising: processingcircuitry configured to: receive a bit stream including bins of syntaxelements, the syntax elements corresponding to coefficients of a regionof a transform skipped block in a coded picture, the syntax elementsincluding a first flag indicating whether an absolute coefficient levelof one of the coefficients is greater than a first threshold, and asecond flag indicating whether the absolute coefficient level of the oneof the coefficients is greater than a second threshold, the secondthreshold being greater than the first threshold; determine a number ofthe bins of the second flag that are context coded based on codedinformation of the coefficients; perform context modeling to determine acontext model for each of the number of the bins of the second flag; anddecode the number of the bins of the second flag based on the determinedcontext models.
 21. The apparatus of claim 20, wherein the codedinformation of the coefficients includes a number of the first flag in acurrent coefficient group (CG) including the coefficients, a number thefirst flag in a previous CG, whether a current block corresponding tothe transform skipped block is intra coded or inter coded, a size of thetransform skipped block, a width of the transformed skipped block, aheight of the transformed skipped block, or an aspect ratio of thetransformed skipped block.
 22. An apparatus of video decoding,comprising: processing circuitry configured to receive a bit streamincluding bins of syntax elements, the syntax elements corresponding tocoefficients included in a coefficient group (CG) of a transform skippedblock in a coded picture; determine a maximum number of context codedbins of the CG or a maximum average number of context coded bins of theCG based on coded information of the coefficients; perform contextmodeling to determine a context model for each of a number of the binsof syntax elements in the CG, the number of the bins of syntax elementsin the CG being context coded not exceeding the maximum number ofcontext coded bins of the CG or the maximum average number of contextcoded bins of the CG; and decode the number of the bins of syntaxelements based on the determined context models.